Plasma chambers commonly are used to perform processes for fabricating electronic devices such as semiconductors, displays and solar cells. Such plasma fabrication processes include chemical vapor deposition of semiconductor, conductor or dielectric layers on the surface of a workpiece or etching of selected portions of such layers on the workpiece surface.
It is important for a plasma fabrication process to be performed with high spatial uniformity over the surface of the workpiece. That is, a deposition process should be performed so that the deposited material has uniform thickness and quality at all positions on the surface of the workpiece. Likewise, an etch process should etch material at a uniform rate at all such positions.
Many designs for improving spatial uniformity of plasma fabrication processes have been developed. Many conventional designs attempt to improve spatial uniformity of the fabrication process by maximizing spatial uniformity of the density of the plasma. Other conventional designs attempt to correct for differences between the center and the perimeter of the workpiece in concentration of reactive species by creating a plasma density that varies as a function of radial distance from the center of the workpiece.
A shortcoming of conventional designs is that they cannot compensate for non-uniformities in the plasma density that are not radially symmetrical as a result of asymmetries in the geometry of the plasma chamber.